
Direct2Drive has revealed they will not be selling Modern Warfare 2, pointing the blame indirectly at Steam.
The digital distributor strongly believes that buyers shouldn't have to be forced to install and run third party software. This would refer to the game's use of Steam for, among other things, automatic updates.
From the Modern Warfare 2's description page on D2D:
Thanks for your interest in Call of Duty: Modern Warfare 2 from Direct2Drive.
At Direct2Drive, we believe strongly that when you buy a game from us, you shouldn't be forced to install and run a 3rd party software client to be able to play the game you purchased. Because COD MW 2 requires you, the consumer, to do that, we aren't able to offer the game via Direct2Drive at this time.
At least the console versions have had fairly smooth sailing...